The above-mentioned L-type suspension arm is widely used as a lower arm, etc. of an automobile, and a conventional structure is known in which, in order to reduce the weight of the suspension arm, the arm main body is formed from an upper half body and a lower half body, the upper half body and lower half body being formed by press forming of a plate material so as to have a U-shaped cross-section, and corresponding opposite side wall parts of the upper and lower half bodies are welded to each other in a state in which openings thereof face each other (ref. Patent Document 1 below).    Patent Document 1: Japanese Patent Application Laid-open No. 2003-19907